This invention relates to linear fluid actuators and more particularly to actuators employing multiple pistons on a single piston rod.
Many products, including aircraft and spacecraft, require actuators to move some device. In the case of aircraft and spacecraft, the device is often a control surface. One type of actuator is a linear actuator which is so named because it produces linear motion. Representative of linear actuators is the hydraulic or fluid cylinder. Fluid cylinders may be single acting in that fluid pressure is applied to only one side of the piston or double acting where pressure is applied to both sides of the piston. Typically, the double acting cylinder is unbalanced in that equal pressure applied to either side of the piston produces unequal forces because of the area of the piston rod which extends from one side of the piston through the cylinder housing. Conventionally, a double acting cylinder is balanced by providing an external tail rod which is nothing more than a piston rod on both sides of the piston protruding from both ends of the cylinder housing. Usually the external tail rod is covered by extending the cylinder housing and the cylinder attach point is provided at the distal end. This makes the cylinder substantially twice as long, as the total stroke is accommodated at both ends of the cylinder. The conventional method of coping with this problem is to provide an internal tail stock which is nothing more than another rod attached to the cylinder end plate, internal to the cylinder, with a bore through the piston and external rod which slips over the internal tail stock.
However, since the two rods are concentrically engaging, one must necessarily be smaller than the other and the effective piston areas are unbalanced by this differential area.
Frequently in air and space craft it is required that the actuator have a dual or second redundant means of actuation. This is generally provided by a single cylinder with a divider at the midpoint to form two compartments with a piston in each compartment and one common rod through both pistons. To balance this actuator an external tail rod is provided, as discussed above. Again, this makes the actuator very long.
A need arose in a critical space craft application to provide a redundant (dual) balanced double acting linear actuator having an overall length no longer than a single balanced double acting linear actuator and capable of installation within the same space envelope, and without change to the mounting structure. The object of this invention is to satisfy these requirements.
Failures in hydraulic cylinders which are highly stressed, other than packing failures, often occur as fatigue cracks in the cylinder housing. It is a further object of this invention to provide a cylinder housing in which a crack cannot propagate from the first cylinder compartment to the second cylinder compartment.
It is a further objective of the present invention to meet the above objectives within the structure of the actuator itself as opposed to complex and less reliable control means or combinations of electrical and hydraulic actuators as have been taught by others.